Process and apparatus for degassing a polymer

ABSTRACT

The present invention relates to a process for stripping residual volatile compounds contained in a thermoplastic polymer. The process comprises (1) forming the polymer in the form of a melt flowing as a main stream, (2) forming a foaming agent in the form of one or more secondary liquid streams, (3) adding the secondary stream(s) to the main stream by spraying so as to divide each secondary liquid stream into several fractional streams and thus to form a polymer melt/foaming agent pre-mixture, (4) introducing the pre-mixture into a static mixer, then into an expansion chamber at reduced pressure so as to separate the polymer melt from the residual volatile compounds and from the foaming agent, and (5) withdrawing the polymer melt from the expansion chamber. The invention also relates to an apparatus for removing residual volatile compounds contained in a thermoplastic polymer. The apparatus comprises (i) a polymer melt feed line, (ii) an addition chamber into which the feed line runs and through which the polymer flows as a main stream, (iii) one or more lines for adding a foaming agent as one or more secondary liquid stream(s), which lines run into the addition chamber and have at their ends a spray device allowing each secondary liquid stream to be divided into several fractional streams, (iv) a static mixer having an inlet, connected to the addition chamber, and an outlet, and (v) an expansion chamber for separating the polymer melt from the residual volatile compounds and from the foaming agent, which chamber is connected to the outlet of the static mixer and is provided with a line for withdrawing the polymer melt and with a line for extracting the residual volatile compounds and the foaming agent.

The present invention relates to a process and to an apparatus fordegassing a polymer, especially in order to remove residual volatilecompounds contained in a thermoplastic polymer.

In a process for manufacturing a polymer, the polymerisation reactionshould in theory be complete and convert all of the monomer intopolymer. In reality, it is known that a polymerisation reaction is nevercomplete, in particular in a solution or bulk polymerisation process,especially because of the increase in the viscosity of thepolymerisation medium during the reaction. Thus, in practice, thepolymer obtained contains in general residual volatile compounds, suchas any monomer that has not reacted, one or more solvents that have beenadded or have built up during the reaction, and oligomers formed duringthe reaction. These products are essentially volatile compounds comparedwith the polymer, which is not volatile. Furthermore, it is known thatthese residual volatile compounds have undesirable effects on thequality of the polymer, such as for example the physical and mechanicalproperties and the toxicological characteristics of the polymer,especially in food packaging applications.

There has long been a need to develop higher-performance and moreeffective processes for removing the residual volatile compoundscontained in polymers, especially because of the ever strictertoxicological regulations. These processes generally consist of apolymer degassing operation called “devolatilisation”. The degassing isgenerally carried out by subjecting the hot polymer, in particular inthe form of a solution or of a melt, to a reduced pressure, preferably apressure below atmospheric pressure (or sub-atmospheric pressure) in oneor more expansion chambers, (also called “flash tanks” or“devolatilisers”), that are placed in series and are under successivelyhigher vacuum levels. In particular, the polymer may be extruded into anexpansion chamber in the form of a melt and divided, for example in theform of “falling strands” so as to facilitate the separation of theresidual volatile compounds from the polymer, which is thus recovered,stripped of these compounds. Such degassing processes are described, forexample, in U.S. Pat. No. 2,970,089, U.S. Pat. No. 3,853,672, U.S. Pat.No. 3,928,300, U.S. Pat. No. 4,294,652, U.S. Pat. No. 4,383,972, U.S.Pat. No. 5,453,158, U.S. Pat. No. 5,540,813 and U.S. Pat. No. 5,874,525.

It is known that improvements have been made over many years in suchdegassing processes, especially by the addition of an inert and volatileagent to the polymer. This agent is generally known by the term “foamingagent”, “blowing agent” or “stripping agent”, or else a“devolatilisation-assisting fluid” or “devolatilisation aid”. Underthese conditions, degassing the polymer generally consists in expandingthe mixture resulting from this addition, when hot and at a reducedpressure, in an expansion chamber such as those described above. Itfollows from this expansion that the foaming agent forms a large numberof bubbles within the polymer melt and that the stripping of theresidual volatile compounds contained in the polymer, by diffusion, isfacilitated by the considerably increased surface area of the foamingmass that results. The foaming agents most often used are thosedescribed in U.S. Pat. No. 3,668,161, U.S. Pat. No. 3,773,740, U.S. Pat.No. 4,195,169, U.S. Pat. No. 4,537,954, U.S. Pat. No. 5,350,813, U.S.Pat. No. 5,380,822 and U.S. Pat. No. 6,410,683. These are generallyliquid fluids under normal conditions and readily volatile under thedegassing conditions, for example water, alcohols or ketones, or asolution of carbon dioxide in water. U.S. Pat. No. 5,691,445 and U.S.Pat. No. 5,861,474 have proposed to replace these conventional foamingagents with a supercritical fluid which would normally be gaseous underthe injection conditions, but which is kept in solution in the polymerthanks to high pressures being applied during the injection. Thus, ithas been proposed, for example, to use nitrogen, carbon dioxide andalkanes, especially C₄ to C₆ alkanes.

In all cases, few details have been given about the way in which thefoaming agent is added to the polymer, except for the fact that it isgenerally recommended to make the addition and then to subject theresulting mixture to a static mixer placed downstream of the expansionchamber. In general, it is believed that the static mixer has the effectof dispersing the foaming agent throughout the polymer mass. However,little has been described about the way in which the foaming agent infact is mixed into and dispersed within the polymer mass. The relativelyhigh temperature and the high pressure that are applied in the processbefore the degassing, and the vacuum and high temperature that areapplied during the degassing in the expansion chamber generally preventdirect obsernations being made on the mixture and on the dispersion ofthe foaming agent in the polymer, the dividing of the foaming agent intorelatively fine liquid particles (or droplets) within the polymer andthe pre-expansion of the polymer, for example in the static mixer.

U.S. Pat. No. 6,124,426 (equivalent to European Patent Application EP905 149 A) proposes the injection of a foaming agent, such as water, analcohol or a ketone, into a polymer melt using a nozzle directed in theopposite direction to the flow of the polymer, this having the effect ofimproving the mixing performance of a static mixer placed downstream ofthe point of addition of the foaming agent. An injection nozzle isgenerally defined as being a line that includes a restriction so as toaccelerate and direct a fluid, the pressure of which drops on leavingthe nozzle. In the United States patent, it is specified that a nozzleoriented in the opposite direction, that is to say in the direction offlow of the polymer, tends to concentrate the foaming agent at one pointin the static mixer and that this results in non-uniform mixing, areduction in the performance of the mixer and finally less effectivedegassing of the polymer.

European Patent Application EP 1 084 739 discloses an apparatus and aprocess for the devolatilisation of polymers, in particular for theremoval of volatile impurities from thermoplastic polymers. The processcomprises charging a stripping agent into a molten polymer at an inletend of a static mixer means via a pump and an injection device. However,the European Patent Application is silent about the injection device andhow the stripping agent is charged into the molten polymer.

U.S. Pat. No. 3,644,296 discloses a process for the high molecularweight polymerisation of lactams. The process comprises (i) mixing afirst portion of lactam monomers with a catalyst to provide a firstcomponent, (ii) mixing a second portion of lactam monomers with apolymerisation accelerating promoter to provide a second component,(iii) mixing the first and second components together in a liquid state,and (iv) introducing the resulting mixture into a polymerisation zone.However, the United States Patent discloses neither a process, nor anapparatus for degassing a polymer and more particularly for removingresidual volatile compounds contained in a polymer after apolymerisation. It does not disclose the addition of a foaming agent toa melted polymer.

U.S. Pat. No. 4,233,269 discloses a fluid flow distributor for mixingand distributing gas and liquid over the cross-section of a reactorvessel having an upward fluid flow path. In the reactor vessel, gasesand liquids are contacted in order for a desired chemical reaction toproceed, e.g. in operations for hydrofininig of oils, hydrocracking ofhydrocarbons into lighter compounds, hydrogenation of olefins andaromatics and oxonation of olefins to aldehydes. However, the UnitedStates Patent describes neither a process, nor an apparatus fordegassing a polymer. In particular, it does not describe the addition ofa liquid to another liquid. Furthermore, it discloses neither a staticmixer, nor an expansion chamber at reduced pressure for separating apolymer melt from residual volatile compounds.

U.S. Pat. No. 6,419,386 proposes a static mixing apparatus comprisingtwo laminar static mixers, placed in series, having a cross section thatincreases in size in the direction of flow of a high-viscosity mainproduct intended to be mixed with a low-viscosity additive. Theapparatus furthermore includes a device for injecting the additive intothe main product. The device comprises a plate having a convergentorifice through which the main product and the additive pass, whichorifice is placed before or at the inlet of the first of the two staticmixers. It also includes a line for injecting the additive (provided atits end with a nozzle) aligned with the orifice and oriented in thedirection of flow of the main product. The nozzle comprises a centralfeed channel which emerges on the axis of the orifice in the plate. Itis specified that the proposed apparatus is suitable for mixing aviscous product, such as a polystyrene, with a much less viscousadditive, such as a mineral oil or a paraffin oil, soluble inpolystyrene. The additive may, in other cases, be a gas such asnitrogen, carbon dioxide or steam. However, it is not mentioned whetherthe apparatus, and especially the injection device, may be suitable foradding a foaming agent to a polymer, and mixing it thereinto, during adegassing operation.

However, it has been attempted to use the injection device described inU.S. Pat. No. 6,419,386, for the purpose of adding a foaming agent, suchas water, into a molten polymer manufactured continuously in a bulkpolymerisation process, such as a polystyrene, at the inlet of a staticmixer placed upstream of an expansion chamber for degassing the polymer.“Pounding” or “hammering” phenomena are then observed, which areaccompanied by substantial vibration in the mixer, with repercussions inthe expansion chamber. The magnitude of these phenomena was such thatthe plant could have been damaged and its safety put into jeopardy.These phenomena appear especially during changes in the manufacture ofthe polystyrene, for example a reduction in the hourly production rateand/or a decrease in the average molecular mass of the polymer. Afterextensive research, it has been found that these pounding phenomenacould be due to “cavitation” phenomena occurring in the mass of thepolymer melt flowing through the static mixer. Such phenomena could becaused by the water injected into and poorly dispersed within thepolymer. In particular, lowering the pressure to below the vapourpressure of the water could lead to sudden vaporization of the water andthe formation of large pockets of water vapour. These gas pockets arecharacterized by instability owing to the fact that, subsequently, theslightest variations in temperature and pressure could lead to thewvater suddenly recondensing.

The object of the present invention is specifically to correct theseshortcomings and allow more effective degassing of polymers, especiallywhen conventional foaming agents are used, in particular liquid fluidsthat are readily volatile and generally insoluble in these polymers. Oneof the advantages of conventional foaming agents derives from the factthat they are injected in liquid form and that the small amountsintroduced are easier to control.

The present invention firstly relates to a process for strippingresidual volatile compounds contained in a thermoplastic polymer,characterized in that it comprises the following steps:

(1) forming the polymer in the form of a melt flowing as a main stream;

(2) forming a foaming agent in the form of one or more secondary liquidstreams;

(3) adding the secondary liquid stream(s) to the main stream by sprayingso as to divide each secondary liquid stream into several fractionalstreams and thus to form a polymer melt/foaming agent pre-mixture;

(4) introducing the pre-mixture into a static mixer, then into anexpansion chamber at reduced pressure so as to separate the polymer meltfrom the residual volatile compounds and from the foaming agent; andwithdrawing the polymer melt, thus stripped of the residual volatilecompounds and of the foaming agent, from the expansion chamber.

FIG. 1 shows schematically an apparatus for degassing a thermoplasticpolymer, allowing the process of the invention to be implemented.

FIGS. 2, 3 and 4 show schematically several embodiments of a spraydevice that can be installed in the apparatus shown in FIG. 1.

FIG. 5 shows schematically a spray nozzle that can be used in thedevices shown in FIGS. 2, 3 and 4.

The thermoplastic polymer used in the process of the invention may be athermoplastic homo- or co-polymer, or a blend of two or morethermoplastic (co-)polymers, especially chosen from olefin polymers,especially poly(alpha-olefins) such as a low-density polyethylene(LDPE), a high-density polyethylene (HDPE), a linear low-densitypolyethylene (LLDPE), a co-polymer of ethylene with at least onealpha-olefin, for example a C₃ to C₈ alpha-olefin, a polypropylene, apolybutene, a polyisobutene, or a blend of a polyethylene with apoly(alpha-olefin). The thermoplastic (co-)polymers may also be chosenfrom vinyl polymers, especially aromatic vinyl polymers, such as apolystyrene, a poly(alpha-methylstyrene), a high-impact polystyrene(HIPS), in particular one modified by grafting on a natural or syntheticrubber, such as a polybutadiene or a polyisoprene, astyrene/acrylonitrile co-polymer (SAN), a styrene/maleic anhydrideco-polymer (SMA), an acrylonitrile/butadiene/styrene ter-polymer (ABS),a styrene/acrylic acid co-polymer, a styrene/methyl methacrylateco-polymer, and a polyvinyl chloride. The thermoplastic (co-)polymersmay also be chosen from polycarbonates, polyamides, polyesters,polysiloxanes and synthetic rubbers, such as a polybutadienle, apolyisoprene, an ethylene-propylene rubber (EPR) and anethylene-propylene-diene rubber (EPDM). It is preferable to choose athermoplastic (co-)polymer from olefin polymers and vinyl aromaticpolymers, such as those mentioned above, and especially from styrene(co-)polymers, such as polystyrene and high-impact polystyrene (HIPS).In the present description, the term “polymer” means both a homo-polymeror co-polymer and a blend of two or more (co-)polymers.

In the process of the invention, the residual volatile compoundscontained in the polymers may in general be one or more residualmonomers, one or more organic solvents that have been added or built upduring the manufacture of the polymers, especially aliphatichydrocarbons, such as hexane, heptane, octane or decane, aromatichydrocarbons, such as benzene, toluene, ethylbenzene, xylene, cumene orother alkyl benzenes, halogenated hydrocarbons, halogenated aromatichydrocarbons, nitrile compounds, amine compounds and also oligomersproduced during the manufacture of the polymers. In particular, theresidual volatile compounds found at the end of the manufacture ofstyrene polymers are essentially residual styrene, hydrocarbonimpurities generally accompanying the monomer, related to inert solventshaving a low boiling point, such as ethylbenzene, cumene,n-propylbenzene, methylcyclohexanie and ethyltoluene, and styreneoligoniers, such as styrene dimer-s and trimers. These residual volatilecompounds are found in the polymers after polymerisation. In general,the residual monomer content, for example the residual styrene content,may be around 0.5 to 25%, preferably 1 to 10%, by weight with respect tothe polymer.

A preliminary degassing step may be carried out on the polymer bysubjecting the polymer melt to reduced pressure, so as to remove asubstantial portion of the residual volatile compounds before theprocess of the invention is carried out. The polymer resulting from thispreliminary step may have a residual monomer, for example residualstyrene, content of around 500 to 5000, preferably 1000 to 3000, partsby weight per million (ppm) with respect to the polymer.

The process of the invention comprises a step in which the polymercontaining the residual volatile compounds, as described above, isformed as a melt. In general, the polymer is heated to a temperatureabove the glass transition temperature T_(g) (measured according to theASTM E 1356-98 method) of the polymer and preferably below thedecomposition temperature of the polymer. The polymer may be heated togasufficiently high temperature, and well above the T_(g) of the polymer,for the viscosity of the polymer not to be too high, for the polymer tobe able to flow relatively easily and for degassing to be facilitated.Thus, the polymer may be heated to a temperature greater than (T_(g)+30°C.), preferably greater than (T_(g)+50° C.), especially greater than(T_(g)+90° C.). The polymer melt may result from melting the polymer,for example in an extruder. Preferably, it may result directly from themanufacture of the polymer, when the polymer is manufactured especiallyusing a solution, or preferably bulk, polymerisation process. Thus, whena styrene polymer is manufactured using a bulk polymerisation process,the polymer is generally obtained, after polymerisation, in the form ofa melt at a temperature ranging from 130 to 200° C., preferably 150 to190° C. The polymer melt is then preferably preheated to a suitabletemperature for degassing the polymer, for example a temperature rangingfrom 180 to 300° C., preferably 200 to 280° C., especially 220 to 260°C. The preheating may be carried out in a heat exchanger, for example ofthe static mixer type. Furthermore, it is preferable to subject thepolymer melt to a prior degassing operation in an expansion chamber, ata temperature that may range from 180 to 300° C., preferably 200 to 280°C., especially 220 to 260° C., and at a reduced pressure, for example apressure below atmospheric pressure, preferably an absolute pressureranging from 5×10² to 5×10⁴ Pa, especially 10³ to 10⁴ Pa. In general,the purpose of the prior degassing operation is essentially to separatea substantial portion of the residual volatile compounds from thepolymer, before the degassing according to the invention which iscarried out using a foaming agent, thus making it possible to optimisethe removal of the residual volatile compounds until a polymer with verylow contents of these compounds is obtained.

The polymer melt is employed in the process of the invention in the formof a main stream, preferably continuously, for example using a gearpump.

The process of the invenition also comprises a step in which a foamingagent is formed in the form of at least one secondary liquid stream. Thefoaming agent may preferably be chosen from fluids that are liquid undernormal conditions and are also readily volatile, especially under thedegassing conditions, in particular the reduced-pressure expansionconditions. Furthermore, the foaming agent may be insoluble (orimmiscible) or substantially insoluble in the polymer melt. The foamingagent may be chosen from water, alcohols, especially C₁ to C₁₀,preferably C₁ to C₅, alcohols, ketones, especially C₃ to C₁₀, preferablyC₃ to C₅, ketones, an aqueous carbon dioxide solution, and mixtures oftwo or more of these products. Preferably, the foaming agent is chosenfrom water, methanol, ethanol, n-propanol, isopropanol, n-butanol,isobutanol, acetone, a 0.1 to 10 wt %, preferably 0.5 to 5 wt %,especially 0.5 to 1.5 wt %, carbon dioxide solution, (based on the totalweight of the solution) and a mixture of two or more of these products.The amount of foaming agent added to the polymer may be 0.1 to 8%,preferably 0.2 to 5%, especially 0.5 to 3% by weight with respect to thepolymer.

The foaming agent is employed in the process of the invention in theform of one or more secondary liquid streams, preferably continuously,using for example one or more pumps.

The process of the invention also includes a step in which the secondaryliquid stream(s) is(are) added, preferably continuously, to the mainstream by spraying so as to divide each secondary liquid stream intoseveral fractional streams. The expression “spraying of the secondaryliquid stream” is understood to mean in general any means for dividingor fragmenting the secondary liquid stream into several fractionalstreams, especially into at least two, or preferably at least three, orespecially at least four fractional streams, for example into a numberof fractional streams ranging from 2 to 20, preferably 3 to 15,especially 4 to 12. This division or fragmentation into severalfractional streams allows the above-mentioned “pounding” phenomena to bereduced or even eliminated, and at the same time allows theeffectiveness of the degassing process to be very considerably improved.One of the observed improvements in the process may, for example, besuch that it is possible to reduce the degassing temperature and/or toreduce the vacuum (that is to say-increase the sub-atmospheric pressure)in the expansion chamber, whilst continuing to remove a constant amountof volatile compounds. Alternatively, the content of residual volatilecompounds in the polymer may be substantially reduced, especiallycompared with the known prior processes. Another observed improvementmay also be such that it is possible to reduce the amount of foamingagent used in the degassing, whilst still continuing to remove aconstant amount of residual volatile compounds. Such a reduction in theamount of foaming agent makes it easier to carry out the subsequentoperations of separating, condensing and recovering the residualvolatile compounds and in particular the residual monomer. The residualmonomer thus recovered may advantageously be returned to thepolymerisation in order to manufacture polymer. To give an example, verysubstantial cost reductions relating to the heating, cooling and/orenergy means employed in these separation, condensation and recoveryoperations may be achieved.

The spraying of the secondary liquid stream into the main stream iscarried out in such a way that the secondary liquid stream is dividedinto several fractional streams, or more particularly into severalliquid jets that can thus penetrate the main stream and can thenthemselves be reduced or subdivided more easily into small liquidparticles or droplets. The spraying is preferably carried out so as toorient the fractional streams in a direction at right angles to thedirection of the main stream or at a right, acute or zero angle thereto,preferably an acute angle or zero angle, that is to say in a directionhaving a non-zero component directed along the direction of the mainstream. In particular, at least one of the fractional streams may bechosen to be oriented in a direction equivalent to the direction of themain stream or substantially in this direction, while at least one ofthe other fractional streams is oriented in a direction making an angleof greater than 20° and less than or equal to 90°, preferably an angleof greater than 20° and less than 90°, for example an angle ranging from30° to 80°, especially 45° to 75°, with the direction of the said mainstream. The expression “direction substantially in the direction of themain stream” may be understood to mean a direction making an angle of20°, preferably ±10, with the direction of the said stream.

When the foaming agent is used in the form of two or more secondaryliquid streams, the latter are preferably introduced simultaneously byspraying them into the main stream and especially in one of thepreferred forms described above.

Such spraying makes it possible to form a pre-mixture in which thepolymer melt is pre-mixed with the foaming agent thus divided orfragmented beforehand. This pre-mixture is especially produced before orjust before the moment when it is introduced into a static mixer.

The temperature and the pressure of the secondary liquid stream(s) arein particular such that the foaming agent is in liquid form at themoment when it is added to the main stream. In particular, the secondaryliquid stream(s) may be at a temperature equal to or preferably lessthan that of the main stream, for example at a temperature ranging fromroom temperature (for example 20° C.) up to 200° C., or preferably up to150° C., and at a pressure greater than that of the main stream, forexample 0.2 to 3 MPa, preferably 0.3 to 2 MPa, greater than that of themain stream.

At the point of addition of the secondary liquid stream(s) into the mainstream, the main stream may be at the same or approximately the sametemperature as that of the polymer melt, in particular at a temperatureas indicated above, especially at the moment of preheating. At thispoint of addition, the main stream may be at an absolute pressure chosenwithin the range from 1 to 12 N4 Pa, preferably 1.5 to 10 MPa,especially 2 to 8 MPa.

According to a preferred version, it may be advantageous to add thesecondary liquid stream(s) to the main stream at the moment when thelatter is subjected to a constriction, or more particularly to a doubleoperation comprising, in the direction of flow of the main stream, insuccession a decompression phase followed by a compression phase. Theconstriction may be obtained by a restriction (or narrowing) of the mainstream produced by, for example, an orifice plate or a “venturi” device.The constriction may comprise, in the direction of the main stream, insuccession an upstream or convergent section followed by a downstream ordivergent section, the narrowest part of the constriction being locatedbetween the convergent section and the divergent section, that is to saybetween the decompression phase and the compression phase. It has beenfound that the pre-mixing is substantially improved when the addition ofthe secondary liquid stream(s) is carried out during the constriction ofthe main stream, and more particularly during the decompression phase orduring the compression phase, or else between these two phases. The bestresults were obtained when the addition is especially carried outbetween the decompression and compression phases, or preferably duringthe compression phase of the main stream. It has in fact been observedthat such an addition has the effect of improving the spraying,especially the division of the foaming agent into fractional streams,and thereafter of making it easier to degas the polymer.

The constriction may be employed in such a way that, during thedecompression phase, the main stream is subjected to a pressure dropranging from 0.2 to 2 MPa, preferably 0.3 to 1.2 MPa, and thatthereafter, during the compression phase, it is subjected to an increasein the pressure, the magnitude of the increase generally being less thanthat of the drop, in particular ranging from 0.1 to 1 MPa, preferably0.1 to 0.5 MPa. The constriction of the main stream may thus cause anoverall head loss ranging from 0.1 to 1 MPa, preferably 0.2 to 0.7 MPa.

When the constriction of the main stream is carried out by applying aconvergent section and a divergent section respectively to the passageof the stream, it is not only advantageous to introduce the secondaryliquid stream(s) between the decompression and compression phases, orpreferably during the compression phase, of the main stream, but it isalso furthermore particularly recommended to orient at least one of thefractional streams in a direction parallel or approximately parallel tothe plane of the divergent section. The expression “directionapproximately parallel to the plane of the divergent section” may beunderstood to mean a direction making an angle of ±20°, preferably ±10°or even ±5°, with the plane of the divergent section. Furthermore, itmay be recommended for at least one of the other fractional streams tobe simultaneously oriented in a direction equivalent to the direction ofthe main stream or substantially in this direction. The expression“direction substantially in the direction of the main stream” may beunderstood to mean a direction making an angle of ±20°, preferably ±10°,with the direction of the said stream.

Moreover, it has been found that forming the pre-mixture under theconditions of the invention then tends to facilitate the operationscarried out in the static mixer and in the expansion chamber, so thatthe process of degassing the polymer is considerably improved overall.

Specifically, the process of the invention then comprises theintroduction of the pre-mixture into a static mixer, the introductionpreferably being carried out continuously. The static mixer may be alinear or conventional static mixer, for example a static mixer of theSMX type sold by Sulzer Chemtech or Koch Glitsch, or anotherconventional static mixer sold by Kenics, Toray or Ross. It is alsopossible to use a static mixer as described in Japanese PatentApplications JP 62-191274 or JP 57-15258, in British Patent ApplicationGB 2 010 739 or in French Patent FR 2 223 073. It is also possible touse a static mixer as described in U.S. Pat. No. 6,419,386, particularlyone suitable for mixing products of very different viscosity: inparticular, it comprises two static mixers in line, each having adifferent cross section. However, the process of the invention has theadvantage of being able to use a more conventional static mixer, such asone of those mentioned above.

The static mixer may operate under conditions such that the shear ratelies within a range from 1 to 200 sir, especially 1 to 1.0 s⁻¹. Thetemperature of the static mixer may be the same as that of the expansionchamber. In particular, it may be chosen within a range from 180 to 300°C., preferably 200 to 280° C., especially 210 to 260° C. or 220 to 245°C. By virtue of the process of the invention, the temperature of thestatic mixer may advantageously be reduced, for example by at least 10or 15° C., compared with that normally used during this step, withoutthereby affecting the homogenisation of the mixing and withoutrestricting the dispersion of the foaming agent within the polymer. Thepressure at the inlet of the static mixer may be chosen within a rangefrom 1 to 12 MPa, preferably 1.5 to 10 MPa, especially 2 to 8 MPa. Thepressure at the outlet of the static mixer may be the same, orapproximately the same, as that in the expansion chamber. According tothe process of the invention, the static mixer may essentially have theeffect of homogenising the pre-mixture and of continuing to disperse anddivide the foaming agent into extremely fine liquid particles ordroplets in such a way that the mixing that results is optimised so asthereafter to be advantageously expanded and degassed. The meanresidence time of the polymer melt and the foaming agent in the staticmixer may range from 0.5 to 20 minutes, preferably 0.5 to 10 minutes,especially 1 to 5 minutes. Advantageously, it may be reduced comparedwith those known in the conventional polymer degassing processes.

The process of the invention then comprises the introduction of themixture into an expansion chamber, this introduction preferably beingcarried out continuously. The expansion chamber may be one of thosedescribed in the polymer degassing processes. In particular, anexpansion chamber may be used in which the mixture is extruded in theform of a divided mass, for example in the form of falling strands. Theabsolute pressure applied in the expansion chamber is preferably chosenin such a way that the mixture can be expanded by vaporizing the foamingagent. A sub-atmospheric pressure (that is to say a pressure belowatmospheric pressure), which may be chosen within a range from 10² to10⁴ Pa, preferably 10² to 5×10³ Pa, especially 5×10² to 5×10³ Pa, mayespecially be applied. The sub-atmospheric pressure applied in theexpansion chamber may advantageously be greater, for example by a factorof at least 2, than that normally used during this step, without therebyimpairing the effectiveness of the degassing. The temperature of theexpansion chamber is preferably chosen in such a way that the mixturecan be expanded and the polymer maintained in the melt, especially in arelatively fluid form, and without being substantially degraded ordecomposed. The temperature may be chosen within a range from 180 to300° C., preferably 200 to 280° C., especially 210 to 260° C. or 220 to245° C. By virtue of the process of the invention, the temperature ofthe expansion chamber may be advantageously reduced, for example by atleast 10 or 20° C., compared with that normally used during this step,without thereby impairing the effectiveness of the degassing. Such areduction in temperature is furthermore particularly beneficial since itmakes it possible to reduce the effects of any thermal de-polymerisationof the polymer and especially to reduce the residual monomer content inthe polymer.

The expansion chamber thus allows the residual volatile compounds andthe foaming agent to be separated from the polymer melt. In particular,the residual monomer content, for example residual styrene content, ofthe polymer degassed according to the invention may be equal to or lessthan 250 ppm, and preferably be in a range from 50 to 250 ppm,especially 50 to 200 ppm.

The polymer, thus substantially stripped of the residual volatilecompounds and the foaming agent, is withdrawn from the expansionchamber, preferably continuously, especially by a gear pump.

The present invention also relates to an apparatus for removing residualvolatile compounds contained in a thermoplastic polymer, especially byimplementing the process described above, which apparatus ischaracterized in that it comprises:

-   -   a polymer melt feed line;    -   an addition chamber into which the feed line runs and through        which the polymer melt flows as a main stream;    -   one or more line(s) for the addition of a foaming agent flowing        as one or more secondary liquid streams, which line(s) runs        (run) into the addition chamber and has (have) at its (their)        end(s) a spray device allowing each secondary liquid stream to        be divided into several fractional streams;    -   a static mixer having an inlet, connected to the addition        chamber, and an outlet; and    -   an expansion chamber for separating the polymer melt from the        residual volatile compounds and from the foaming agent, which        chamber is connected to the outlet of the static mixer and is        provided with a line for withdrawing the polymer melt thus        separated and with a line for extracting the residual volatile        compounds and the foaming agent.

The apparatus according to the invention comprises an addition chamberinto which a line for feeding the polymer melt containing especially theresidual volatile compounds to be removed runs and through which thepolymer melt flows as a main stream. Running into the addition chamberare one or more lines for the addition of a foaming agent flowing as oneor more secondary liquid streams. The apparatus also comprises a staticmixer having an inlet connected, in particular directly or indirectly,to the addition chamber and an outlet connected, in particular directlyor indirectly, to an expansion chamber. The thermoplastic polymer andthe foaming agent may especially be those described above.

The addition chamber may be of any form. In particular, it may be in theform of a line or pipe, in particular a line that extends the polymermelt feed line, or else a chamber contiguous with (or adjacent to) theinlet of the static mixer, and preferably placed on the longitudinalaxis of the static mixer. The addition chamber may especially bedesigned to withstand the relatively high pressures and temperatures,such as those indicated above.

The addition line(s) run into the addition chamber and have, at theirend, a spray device for dividing each secondary liquid stream intoseveral fractional streams, especially so as to form in the additionchamber a pre-mixture of the polymer melt with the foaming agent thuspre-divided or pre-fragmented. Dividing the secondary liquid stream intoseveral fractional streams was described in detail above. The spraydevice may be any system capable of mechanically dividing a liquid mass.In particular, it may be chosen from sprayers, atomizers, vaporizers ornebulization devices. It may especially consist of a closed nozzle,placed at the end of the addition line and pierced by several orifices,the number of which is equivalent to the number of fractional streams tobe formied, in particular at least 2, preferably at least 3 or 4, pernozzle, for example a number ranging from 2 to 20, preferably 3 to 15,especially 4 to 12 per nozzle. The nozzle may have any shape and inparticular be in the shape of a closed envelope pierced with orifices,having an open base, especially one contiguous with the end of theaddition line. The nozzle may in particular have the shape of acylindrical envelope, especially an envelope cylindrical of revolution,such as a hollow plug, one of the two bases of which is closed and theother base is open and especially contiguous with the end of theaddition line, both the envelope and the closed base being pierced withorifices. The nozzle (or in particular its envelope) may have a wallsuch that each pierced orifice consists of a channel, preferablystraight, passing right through the wall. The orifice or the crosssection of the channel is generally circular and may have a diameterranging from 0.1 to 10 mm, preferably 0.5 to 5 mm, for example 1 to 3mm. The nozzle has especially orifices (or, in particular, channelspassing through the wall of the nozzle) that are oriented in such a waythat the resulting fractional streams are directed in a directionmaking, with the direction of the main stream flowing through theaddition chamber, a right, acute or zero angle, preferably an acute orzero angle, that is to say in a direction having a non-zero componentdirected along the direction of the said stream. In particular, it ispossible to choose a nozzle at least one of the orifices (or channels)of which is directed in such a way that the resulting fractional streamis oriented in a direction equivalent to the direction of the mainstream flowing through the addition chamber or substantially in thisdirection, while at least one of the other orifices (or other channels)is directed in such a way that the resulting fractional stream isoriented in a direction making an angle of greater than 20° and lessthan or equal to 90°, preferably an angle of greater than 20° and lessthan 90°, for example an angle ranging from 30 to 80°, especially 45 to75°, with the direction of the said main stream. The expression“direction substantially in the direction of the main stream” may beunderstood to mean a direction making an angle of ±20°, preferably ±10°,with the direction of the said stream. The orifices or channels orientedin a direction equivalent to the direction of the main stream orsubstantially in this direction may have a diameter the same as ordifferent from that of the differently oriented orifices or channels:preferably, they may have a diameter greater, for example 1.2 to 4times, preferably 1.5 to 3 times, greater than that of the differentlyoriented orifices or channels.

According to a preferred variant, the spray device is placed in theaddition chamber which may have a constriction zone (or restriction ornarrowing) so that the main stream undergoes a constriction as describedabove. The constriction zone may comprise, in the direction of flow ofthe main stream, in succession an upstream or convergent section and adownstream or divergent section, the narrowest part of the zone beinglocated between the two sections. The spray device may be placed at anypoint in the constriction zone, for example in the upstream orconvergent section or in the downstream or divergent section, or else inthe narrowest part of the zone, that is to say located between the twosections. It has been noted that the best results are obtained when thespray device is located in the narrowest part of the constriction zone,or preferably in the downstream or divergent section of the zone. Theplane of the divergent section may make an acute angle (A) or rightangle, more specifically an angle of greater than 20° and less than orequal to 90°, preferably an angle of greater than 20° and less than 90°,for example an angle ranging from 30° to 80°, especially 45° to 75°,with the direction of the main stream flowing through the additionchamber, whereas the plane of the convergent section may make an obtuseangle (B) or right angle, more specifically an angle of greater than orequal to 90° and less than 160°, preferably an angle of greater than 90°and less than 160°, for example ranging from 100° to 150°, especially105° to 135°, with the direction of the said main stream. The angles (A)and (B) are shown in particular in FIG. 3. The constriction zone may,for example, consist of an orifice plate or a “venturi” device. Thenarrowest part of the constriction zone may correspond to a largereduction in the area of the cross section for flow of the main streamin the addition chamber: the area may be reduced by a factor of at least2, preferably at least 5, especially at least 10, for example by afactor ranging from 2 to 150, preferably 5 to 120, especially 10 to 80.

When the spray device is located in the narrowest part of theconstriction zone or preferably in the downstream or divergent sectionof this zone, it is possible to choose a spray nozzle which has at leastone of its orifices (or channels passing through the wall of the nozzle)directed in such a way that the resulting fractional stream is orientedin a direction parallel or approximately parallel to the plane of thedownstream or divergent section. The expression “direction approximatelyparallel to the plane of the downstream or divergent section” may beunderstood to mean a direction making an angle of ±10°, preferably ±5°,with the plane of the said section. Furthermore, at least one of theother orifices (or other channels) may be directed in such a way thatthe resulting fractional stream is oriented in a direction equivalentto'the direction of the main stream flowing through the addition chamberor substantially in this direction. The expression “directionsubstantially in the direction of the main stream” may be understood tomean a direction making an angle of ±20°, preferably ±10°, with thedirection of the said stream.

The apparatus according to the invention furthermore comprises a staticmixer having an inlet and an outlet, and especially a longitudinal axis.The addition chamber is connected directly or indirectly to the staticmixer. The addition chamber is preferably contiguous with (or adjacentto) the inlet of the static mixer and especially placed on thelongitudinal axis of the static mixer. The static mixer may be a linearstatic mixer and be chosen from conventional static mixers, as describedabove, comprising especially static, possibly heating or cooling, mixingelements.

The apparatus according to the invention also includes an expansionchamber connected to the outlet of the static mixer. The expansionchamber may be chosen from the expansion chambers used in polyimerdegassing processes, such as the expansion chambers mentioned above. Ingeneral it is provided with an extrusion device for dividing the mixtureleaving the static mixer, for example in the form of “falling strands”.The function of the expansion chamber is to separate the polymer fromthe residual volatile compounds and from the foaming agent. It isprovided with a line for extracting the residual volatile components andthe foaming agent. The extraction line may leave the upper portion ofthe expansion chamber and be connected especially to a vacuum pump. Theexpansion chamber is also provided with a line for withdrawing thepolymer melt thus stripped of the residual volatile compounds and of thefoaming agent. The withdrawal line may be provided with a gear pump.

The present invention also relates to the use of the apparatus asdescribed above in a process for degassing a thermoplastic polymer. Theprocess may comprise the steps described above.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows schematically an apparatus for degassing a thermoplasticpolymer allowing the process of the invention to be implemented. Theapparatus comprises a polymer melt feed line (1) provided with a gearpump (2). The line (1) runs into an addition chamber (3) through whichthe polymer melt flows as a main stream. A line (4) for adding a foamingagent-flowing as a secondary liquid stream enters the chamber (3) andhas, on the end of it, a spray device (5) for dividing the secondaryliquid stream into several (in FIG. 1, into three) fractional streams(6) and (6′). The apparatus comprises a static mixer (7) having an inlet(8) and an outlet (9). The inlet (8) is connected to the additionchamber (3) via a line (10). The outlet (9) is connected to an expansionchamber (11) via a line (12), which line (12) enters the upper portion(13) of the expansion chamber and has, on the end of it, a means (14)for dividing and extruding the polymer melt/foaming agent mixture. Theupper portion (13) of the expansion chamber is provided with a line (15)for extracting the residual volatile compounds and the foaming agent.The line (15) is connected to a vacuum pump (16). The lower portion (17)of the expansion chamber (11) is provided with a line (18) forwithdrawing the polymer melt stripped of the residual volatile compoundsand of the foaming agent. The line (18) is provided with a gear pump(19).

FIG. 2 shows schematically a first embodiment of a spray device that canbe used in the apparatus shown in FIG. 1. The elements in FIG. 2identical to those shown in FIG. 1 are identified by the same numericalreferences. A polymer melt feed line (1) runs into an addition chamber(3) through which the polymer melt flows as a main stream (20). Theaddition chamber (3) has the form of a line with the same cross sectionas the feed line (1) and placed in the continuation of the latter. Aline (4) for adding a foaming agent flowing as a secondary liquid stream(21) has, on the end of it, a spray device (5) for dividing thesecondary liquid stream (21) into several (in FIG. 2, into three)fractional streams (6) and (6′). The fractional stream (6′) is orientedin a direction equivalent to the direction of the main stream (20),whereas the other two fractional streams (6) are oriented in a directionmaking an angle of 600 with the direction of the said main stream. Theaddition chamber (3) is connected directly to the inlet (8) of a staticmixer (7), like that shown in FIG. 1.

FIG. 3 shows schematically a second embodiment of a spray device thatcan be used in the apparatus shown in FIG. 1. The elements of FIG. 1that are identical to those shown in FIG. 1 or 2 are identified by thesame numerical references. The polymer melt feed line (1) runs into anaddition chamber (3) through which the polymer melt flows as a mainstream (20). A line (4) for adding a foaming agent flowing as asecondary liquid stream (21) enters the addition chamber (3). The line(4) has, on the end of it, a spray device (5) for dividing the secondaryliquid stream (21) into several (in FIG. 3, into three) fractionalstreams (6) and (6′). The addition chamber (3) has a constriction (22)comprising an upstream or convergent section (23) and a downstream ordivergent section (24). The plane of the upstream or convergent sectionand that of the downstream or divergent section make an angle (B) of1200 and an angle (A) of 600 with the direction of the main stream (20),respectively. The spray device (5) is located in the downstream ordivergent section (24) of the constriction. As a result, the fractionalstream (6′) from the spray device (5) is oriented in a directionequivalent (or parallel) to the direction of the main stream (20),whereas the other two fractional streams (6) are oriented in a directionparallel to the plane of the downstream or divergent section (24) of theconstriction, that is to say in a direction making an angle of 60° withthe direction of the said main stream. The addition chamber (3) runsdirectly into the inlet (8) of a static mixer (7), as shown in FIG. 1.The addition chamber (3) has the same cross section as the static mixer(7).

FIG. 4 shows schematically a third embodiment of a spray device whichcan be used in the apparatus shown in FIG. 1. The elements of FIG. 4that are identical to those shown in FIG. 1, 2 or 3 are identified bythe same numerical references. The device of FIG. 4 is identical to thatshown in FIG. 3, except for the fact that it comprises two lines (4) foradding a foaming agent instead of one line. Thus, the foaming agentflows as two secondary liquid streams (21). Each line (4) has, on theend of it, a spray device (5) for dividing each secondary liquid stream(21) into several (in FIG. 4, into three) fractional streams (6) and(6′). The addition chamber (3) has a constriction (22) comprising anupstream or convergent section (23) and a downstream or divergentsection (24). The plane of the upstream or convergent section (23) andthat of the downstream or divergent section (24) make an angle (B) of120° and an angle (A) of 600 (these angles not being shown in FIG. 4)with the direction of the main stream (20), respectively. The spraydevices (5) are located in the downstream or divergent section (24) ofthe constriction. As a result, the fractional stream (6′) from eachspray device is oriented in a direction equivalent (or parallel) to thedirection of the main stream (20), whereas the other two fractionalstreams (6) are oriented in a direction parallel to the plane of thedownstream or divergent section (24) of the constriction, that is to sayin a direction making an angle of 60° with the direction of the saidmain stream. The addition chamber (3) runs directly into the inlet (8)of the static mixer (7), as shown in FIG. 1. The addition chamber (3)has a cross section identical to that of the static mixer (7).

FIG. 5 shows schematically a spray nozzle that can be used in one of thedevices shown in FIG. 2, 3 or 4. The elements of FIG. 5 that areidentical to those shown in FIG. 2, 3 or 4 are identified by the samenumerical references. A line (4) for adding a foaming agent flowing as asecondary liquid stream (21) enters an addition chamber (3) (not shownin FIG. 5) through which a polymer melt flows as a main stream (20). Theline (4) is provided, on the end of it, with a spray device (5) forminga nozzle having the shape of a cylindrical hollow plug having one baseclosed and the other open and contiguous with the end of the line (4).The nozzle is more particularly formed by a cylindrical envelope (25)having an axis of revolution (26) parallel to the main stream (20), anopen base (27) contiguous with the end of the line (4) and a closed base(28). The cylindrical envelope (25) and the closed base (28) are piercedby two cylindrical channels (29) and by a cylindrical channel (30),respectively, in such a way that the secondary liquid stream (21) isdivided into several (in FIG. 5, into three) fractional streams (6) and(6′). The cylindrical channels (29) are oriented in such a way thattheir axis (coincident with the direction of the fractional streams))makes an angle of 600 with the direction of the main stream (20). Thecylindrical channel (30) is oriented in such a way that its axis(coincident with the direction of the fractional stream (6′)) isidentical (or parallel) to the direction of the main stream (20). Thefollowing examples illustrate the present invention.

EXAMPLE 1

A high-impact polystyrene (HIPS), hereafter called “the polymer” wascontinuously degassed. The polymer was modified by grafting apolystyrene onto a polybutadiene and prepared by a continuous bulkpolymerization process. The polymer contained 94.5% by weight ofpolystyrene and 5.5% by weight of polybutadiene. It had a weight-averagemolecular mass of 210 000 daltons and a residual styrene content of 1700ppm.

The degassing was carried out continuously in an apparatus as shown inFIG. 1. The polymer was used in the form of a melt flowing continuouslyas a main stream with a flow rate of 7000 kg/h, at a temperature of 238°C. and an absolute pressure of 2.5 MPa, in a feed line (1) provided witha gear pump (2). The polymer melt fed an addition chamber (3) throughwhich the polymer melt flew as a main stream (20). Two water additionlines (4), as shown in FIG. 4, entered the addition chamber (3), inwhich lines water heated to 150° C. and at an absolute pressure of 3.5MPa flew as two secondary liquid streams (21). Each line (4) wasprovided, at the end of it, with a spray device (5) identical to thatshown in FIG. 5, except for the fact that the number of cylindricalchannels (29) was 4 instead of 2. Thus, the nozzle (5) forming the spraydevice was pierced by four cylindrical channels (29) 1 mm in diameterand by a cylindrical channel (30) 2 mm in diameter, having directionsand making angles as shown in FIG. 5. It followed that each secondaryliquid stream (21) was divided into five fractional streams (6) and(6′). The total head loss created by the spray device was 0.5 MPa. Thewater flew continuously via the two lines (4) and the spray devices (5)into the addition chamber (3), in a total amount equivalent to 1.5% byweight with respect to the polymer. The addition chamber (3) had aconstriction zone (22), as shown in FIG. 4, having a convergent section(23) and a divergent section (24) making an angle (B) of 1200 and anangle (A) of 600 with the direction of the main stream (20),respectively.

The pre-mixture, resulting from the addition of water to the polymermelt, then flew continuously directly into the inlet (8) of a staticmixer (7) of the SMX® type sold by Koch Glitsch (Switzerland), at atemperature of 235° C. and at an absolute pressure of 2.5 MPa. The meanresidence time of the polymer/water mixture in the static mixer (7) was3 minutes. A polymer melt/water mixture at a temperature of 230° C. andat an absolute pressure of 3.5×10³ Pa was obtained at the outlet (9) ofthe static mixer (7).

The mixture was then introduced continuously into an expansion chamber(11) via a line (12), which line entered the said chamber and had, onthe end of it, an extrusion and dividing means (14) placed in the upperportion (13) of the expansion chamber (11). The expansion chamber (11)was heated to a temperature of 225° C., at an absolute pressure of3.5×10³ Pa. The residual volatile compounds, such as the residualstyrene, and the water were separated from the polymer melt andwithdrawn continuously from the expansion chlamber (11) via anextraction line (15) connected to a vacuum pump (16). The polymer melt,thus stripped of the residual volatile compounds and of the water, wascontinuously withdrawn at 235° C. from the expansion chamber (11) via awithdrawal line (18) provided with a gear pump (19). The residualstyrene content of the polymer thus withdrawn was 150 ppm.

EXAMPLE 2 Comparative Example

Here the process was exactly as in Example 1, except that the water wasintroduced into the addition chamber (3) via the addition lines (4) thatdid not have the spray device (5) on the end of them.

Under these conditions, “pounding”, in particular “hammering”, phenomenawere observed throughout the static mixer (7) and right into theexpansion chamber (11). Moreover, the polymer withdrawn via thewithdrawal line (18) had a residual styrene content of 250 ppm. These“pounding” phenomena put the degassing plant into jeopardy, so that thedegassing process had to be quickly stopped for safety reasons.

1. A process for removing residual volatile compounds contained in a thermoplastic polymer comprising the following steps: (1) forming a thermoplastic polymer containing residual volatile compounds in the form of a melt flowing as a main stream; (2) forming a foaming agent in the form of one or more secondary liquid streams; (3) adding the one or more secondary liquid streams to the main stream by spraying so as to divide each secondary liquid stream into several fractional streams and form a polymer melt/foaming agent pre-mixture; (4) introducing the pre-mixture into a static mixer, then into an expansion chamber at reduced pressure so as to separate the polymer melt from the residual volatile compounds and from the foaming agent; and (5) withdrawing the polymer melt, separated from the residual volatile compounds and the foaming agent, from the expansion chamber.
 2. The process according to claim 1, wherein the thermoplastic polymer is chosen from olefin polymers and aromatic vinyl polymers.
 3. The process according to claim 1 or 2, wherein the foaming agent is chosen from water, alcohols, ketones, an aqueous carbon dioxide solution, and mixtures of two or more of these agents.
 4. The process according to claim 1, wherein each secondary liquid stream is divided, by spraying, into at least two fractional streams.
 5. The process according to claim 1, wherein each fractional stream is oriented in a direction making a right, acute or zero angle with respect to the direction of the main stream.
 6. The process according to claim 1, wherein at least one fractional stream is oriented in a direction substantially equivalent to the direction of the main stream, while at least one other fractional stream is oriented in a direction making an angle of greater than 20° and less than or equal to 90° to the direction of the main stream.
 7. The process according to claim 1, wherein each secondary liquid stream is added to the main stream at the moment when the main stream is subjected to a constriction that comprises, in succession in the direction of flow of the main stream, a decompression phase followed by a compression phase.
 8. The process according to claim 7, wherein each secondary liquid stream is added to the main stream between the decompression phase and the compression phase.
 9. An apparatus for removing residual volatile compounds contained in a thermoplastic polymer, comprising: a thermoplastic polymer melt feed line; an addition chamber into which the feed line runs and through which a thermoplastic polymer melt containing residual volatile compounds flows as a main stream; one or more lines for the addition of a foaming agent flowing as one or more secondary liquid streams, which lines run into the addition chamber and have at their ends a spray device allowing each secondary liquid stream to be divided into several fractional streams; a static mixer having an inlet, connected to the addition chamber, and an outlet; and an expansion chamber for separating the polymer melt from the residual volatile compounds and from the foaming agent, which chamber is connected to the outlet of the static mixer and is provided with a line for withdrawing the polymer melt thus separated and with a line for extracting the residual volatile compounds and the foaming agent.
 10. The apparatus according to claim 9, wherein the spray device includes a closed nozzle placed on the end of the addition line and pierced by several orifices, the number of which is equivalent to the number of fractional streams to be formed by the spray device.
 11. The apparatus according to claim 10, wherein the number of orifices per nozzle is at least
 2. 12. The apparatus according to claim 10 or 11, wherein the orifices are oriented in such a way that the resulting fractional streams are directed along a direction making a right, acute or zero angle, with respect to the direction of the main stream flowing through the addition chamber.
 13. The apparatus according to claim 10 wherein at least one of the orifices is directed in such a way that the resulting fractional stream is oriented in a direction substantially equivalent to the direction of the mainstream flowing through the addition chamber, while at least one other orifices is directed in such a way that the resulting fractional stream is oriented in a direction making an angle of greater than 20° and less than or equal to 90° to the direction of the main stream.
 14. The apparatus according to claim 9, wherein the addition chamber comprises a constriction zone having, in the direction of flow of the main stream, in succession an upstream or convergent section and a downstream or divergent section, the narrowest part of the constriction zone being located between the two sections.
 15. The apparatus according to claim 14, the wherein the spray device is placed in the narrowest part of the constriction zone.
 16. The apparatus according to claim 9, wherein the addition chamber is contiguous with the inlet of the static mixer.
 17. The process of claim 2, wherein the thermoplastic polymer is a styrene (co-)polymer.
 18. The process of claim 3, wherein the alcohols are C₁ to C₁₀ alcohols and the ketones are C₃ to C₁₀ ketones.
 19. The process of claim 4, wherein each secondary liquid stream is divided into at least three fractional streams.
 20. The process of claim 19, wherein each secondary liquid stream is divided into at least four fractional streams.
 21. The process of claim 5, wherein each fractional stream is oriented in a direction making an acute or zero angle with respect to the direction of the main stream.
 22. The process of claim 6, wherein the angle is greater than 20° and less than 90°.
 23. The process of claim 22, wherein the angle is from 30° to 80°.
 24. The process of claim 7, wherein each secondary liquid stream is added to the main stream while the main stream is in the compression phase.
 25. The apparatus of claim 11, wherein the number of orifices per nozzle is at least
 3. 26. The apparatus of claim 25, wherein the number of orifices per nozzle is at least
 4. 27. The apparatus of claim 12, wherein the orifices are oriented in a direction making an acute or zero angle with respect to the direction of the main stream flowing through the addition chamber.
 28. The apparatus of claim 13, wherein the angle is greater than 20° and less than 90°.
 29. The apparatus of claim 28, wherein the angle is from 30° to 80°.
 30. The apparatus of claim 14, wherein the spray device is placed in the downstream or divergent section of the constriction zone. 